The invention relates to a method for nondestructive, three-dimensional detection of structural elements in structures, especially those made of concrete or similar materials.
A method of this kind is known from DE-A-43 20 473 according to which the echo signals from the far field of a single ultrasonic test head can be detected. The amplitudes and travel times of the echo signals can be stored together with the position of the ultrasonic test head in a memory unit as travel time-location curves with the corresponding amplitude values. From the totality of the travel time-location curves, by means of a filter device that has an image-producing device located upstream of it, selectable portions of the stored location curves can be filtered out.
In ultrasonic test methods, sound waves are regularly emitted in the lower frequency range and the multiple reflections of sound waves between a transmitting and/or receiving point and the object to be measured are utilized as the measured parameter. This produces significant peaks in frequency graphs from which the distance to the object can be calculated, if the speed of the waves is known. In the publication "Schickert, G. (editor): Papers and Reports of the International Symposium on Nondestructive Testing in Construction, Report Volume 21, Berlin: DGZfP 1991, pp. 488-504; Wustenberg H., Possibilities and Concepts for Ultrasonic Test Heads, Especially for Construction," regarding the use of ultrasonic echo methods on structures made of concrete, the use of an artificial aperture was proposed. The goal is to eliminate the problems that result from the use of large-area ultrasonic heads by moving small comparatively simply constructed test heads and processing the signals accordingly. For this purpose, an illuminating test head was coupled to the underside of a concrete block and a shadow caused by refraction was produced by means of a bore. As a result of numerical storage of the sound signals recorded with amplitude and phase and of reconstruction using the algorithms of holography, the approximate size of the disturbing object could be determined. However, it is disadvantageous that significant phase errors can be produced due to the non-constant coupling, so that evaluation based on phase-sensitive scanning is unreliable.
In addition, electromagnetic radar methods are employed based on the idea that layers with different dielectric properties are present in the material to be tested. A method of this kind is known from the publication "Symposium on Nondestructive Testing in Construction, Feb. 27, 1991 to Mar. 1, 1991 in Berlin, Report Volume 21, Part 2, pp. 537-544; Author: Dipl.-Ing. C. Florher, Hochtief, Frankfurt/Main; Dipl.-Ing. B. Bernhardt, Berlin, The Locations of Stress Reinforcements Beneath Multilayer Reinforced Concrete Reinforcements." Using an antenna, pulses were transmitted and received primarily in the frequency range between 900 MHz and 2 GHz. The depth information was obtained from the travel-time measurement of the reflected signals, with the propagation rate being calculated from the speed of light in a vacuum divided by the root of the average dielectric constant of the material being investigated. Typical values for the dielectric constant for example are 7 for concrete, 4 for brick, 81 for water, and infinity for iron. Since the value of the dielectric constant is also dependent on moisture, evaluation requires an expert and experienced specialist. In the radar method an antenna is moved continuously, and accordingly a manipulator is used in the ultrasonic test method that is based on the principle of the synthetic aperture. On the basis of these common features, a depth-dependent representation of the object under investigation is made possible by arranging the recorded amplitudes in series. From the different travel times of the signals as well as the dielectric constants of the materials, the depth position of the imaged object is estimated, but the accuracy of the location depends largely on the moisture content of the material or on cavities filled with water, among other factors.
In addition, from the publication "Acoustical Imaging, Vol. 19, edited by Helmut Ermert and Hans-Peter Harjes, Plenum Press, New York and London; Authors: Schmitz, V.; Muller W.; Schafer G.; Synthetic Aperture Focusing Technique--State of the Art" an imaging method is known called "synthetic aperture focusing technique, or SAFT. This imaging method makes it possible to calculate the respective spatial image of an area under investigation.
Finally, a method for measuring the thickness of dielectric objects is known from SU-A-1 364 868. According to this method, electromagnetic waves and sound waves are directed alternatively at the object and the thickness of the object is calculated as a function of the reflected energy.